Bacterial Leaf Blight
Bacterial leaf blight, caused by the pathogen Xanthomonas tardicrescens, affects bearded, Siberian and Japanese irises. The upper leaves develop large, brown blotches, ultimately turning the tips brown. Once your plant has contracted this disease, there is no cure: your best course is to keep your garden free of debris and trim away the diseased parts of your iris. Disinfect your tools after use, so you don't spread the disease.
Fungal Leaf Spot
According to the Maine and Connecticut Iris Society, fungal leaf spot occurs most often in wet conditions. Among the victims of this disease are bearded, Siberian and Dutch irises. The symptoms start at the leaf tip and spread downward. Subsequently, small, circular or oval spots of reddish-brown color appear on the leaves. The disease may be confused with bacterial leaf blight, but, unlike leaf blight, fungicide is effective against fungal leaf spot. Cut out the affected parts of your iris plants, and remove any rotting debris from your iris patch.
Bacterial Soft Rot
Bacterial soft rot in irises is caused by Erwinia carotovora, a fungus that also affects carrots; this problem may simply be referred to as root rot. If left unchecked, this disease can wipe out your bearded irises.
The rhizome will rot, emitting a foul smell; leaves will then wilt and die. Bacterial soft rot thrives in wet, warm springs and in overwatered iris beds.
To fix the problem, dig up the plant, scrape out all affected tissue and expose the rhizome to the sun. Wash the rhizome in antibacterial soap.
Fungal Crown Rot
Save for, perhaps, its flowers, no part of the iris is exempt from rots. Fungal crown rot is caused by Sclerotium rolfsii, also known as mustard seed fungus. This rot affects bearded irises, Japanese, Siberian and spuria irises. The bases of leaves and the earth around affected plants develop a fluffy gray or tan mold; rhizomes are also affected. The leaf tips turn yellow and then rot at the base.
You can prevent fungal crown rot by maintaining a clean garden. If the disease does strike, dig up the rhizomes and treat them in the same way as for bacterial soft rot.
According to the University of Minnesota Extension, the iris borer moth, Macronoctua onusta, is the iris' worst enemy. Moths lay their eggs on old iris leaves and debris; the caterpillars crawl inside and chew their way down to the rhizomes. You'll see lots of little holes and tan streaks. Leaf tips turn brown. By mid-summer, the caterpillars have reached the rhizome. They consume the rhizome, which can kill the plant, and also leave the way open for soft rot infection.
Clean gardens are a deterrent to the iris borer. Other suggestions from the University of Minnessota Extension are to plant resistant flowers, such as Siberian irises. If you find the grubs early, you can manually pick them out and crush them. If you can time the application for when the eggs are hatching, you may also apply an insecticide. Another, more natural, approach is to introduce beneficial nematodes into the earth.